Storage of normally gaseous material in underground caverns



Apnl 2, 1963 P. F. DoUGHl-:RTY 3,083,537

STORAGE 0E NORMALLY GAsEous MATERIAL 1N UNDERGROUND cAvERNs Filed Jan. 25, 1961 ATTORNEY Patented Apr. 2, 1953 33,537 SGRAGE F NQRMALLY GASEQUS MATERiL EN UNDEEEGRUIID SAVERNS Patrick F. Dougherty, Chester Heights, Pa., assigner to Sun @il Company, Philadelphia, Pa., a corporation of New Jersey Filed Sian. 23, 1961, Ser. No. 83,995 4 Claims. (Cl. ol-) rilhis invention relates to the storage of normally gaseous material in yunderground caverns land more particularly concerns the use of underground caverns for storing in liquid phase normally gaseous water-insoluble material which has a critical temperature below the normal wall temperature of the underground cavern. The invention has particular utility in the storage of ethylene in liquid form.

Underground caverns which have been prepared by mining out rock from a solid rock formation or by washing out salt `from a salt deposit have been used for the storage of hydrocarbons. Such Caverns generally are produced at depths ranging `from 3G() to 3000 feet below the ground level and the normal temperature of the cavern wall typically is in the range of 60-85 F. depending upon the depth of the cavern. When the hydrocarbon material has a critical temperature above the normal wall temperature, it is stored in the cavern as a liquid. Thus normally gaseous hydrocarbons such as propane and butane conventionally are stored in underground Caverns under surticient pressure to maintain the hydrocarbon in liquid phase, since their critical temperatures are above the normal wall temperature, However ethylene, which has a critical temperature of about 49 F., heretofore has had to be stored as a gas, so that the amount that can be stored in a cavern of a given size is considerably less than in the case of propane or butane.

Storage Caverns which have been mined in granite or other roel; deposits often have issures in the cavern wall through which water seepage occurs. In some cases where the light hydrocarbon is being stored in vapor phase, this water seepage may cause difficulties due to the formation of hydrocarbon hydrates which are solid and which may cause plugging of the hydrocarbon outlet line. It is desirable 'to store the light hydrocarbon under such conditions that hydrates cannot be formed.

The present invention provides a method of storing a light hydrocarbon such as ethylene as a liquid in underground Caverns having a normal wall temperature which is above the critical temperature of the hydrocarbon. The conditions or" storage iare such that any seepage of water into the cavern is prevented and no hydrocarbon hydrates can ybe formed. While the invention is particularly applicable to the storage of ethylene, it can also be used for storing other water-insoluble normally gaseous materials that have fa critical temperature below the normal wall temperature of the cavern, for example, silicon teh-ahydride or Xenon. it can also be used for the liquid phase storage of ethane where the normal cavern wall temperature is above the critical temperature of this hydrocarbon (90 E).

The method according to the invention comprises initially iilling the cavern lwith a cold aqueous salt solution having a freezing point substantially below the critical temperature of the material to be stored and then circulating the cold salt solution through the cavern Iuntil the Wall temperature approaches that of the salt solution. Preferably the temperature of the salt solution is below 32 F. so that any water in ssures in the cavern Wall will become frozen. This prevents leakage of water into the cavern which otherwise would occur when the hydrostatic head of the water is greater than the pressure in the cavern. After lthe cavern walls have Ibeen suhiciently chilled, the

normally gaseous substance to be stored, which has been liquefied and cooled to about the same temperature as that of the cavern wall, is pumped into the cavern thereby displacing salt solution which iows from the cavern through an outlet line. However, displacing of the salt solution is stopped before its level has reached the bottom of the cavern, so that a body of salt solution is left in the cavern. Thereafter cold salt solution is circulated into the cavern and an equivalent amount of salt solution is withdrawn therefrom. This maintains the cavern at the desired low temperature level so that the stored material will rem-ain in liquid phase. Whenever it is desired to remove stored material lfrom the cavern, the withdrawal of sal-t solution is stopped while the pumping or" cold salt solution into the cavern is continued so that the stored material will be displaced therefrom.

The invention is described more specifically in conjunction with `the accompanying drawing which schematically illustrates the system used in practicing the present method of storage. ln the drawing numeral l() represents an -underground cavern which has been prepared either by inining out a rock formation or washing salt from a rock salt deposit. Assuming that the cavern is in la rock formation such as granite, there are apt to be small iissures in the rock `which normally would allow water to seep into the cavern. An inlet line ll having a valve 12 and colnected to a pump 13 is provided for introducing cold salt solution to the cavern, and an outlet line 14 having valve l5 and `connected to pump -16 is provided for withdrawal of salt solution. Another line `17 connected to pump i8 and to a branch line l@ having valve Ztl also leads to the cavern and terminates adjacent its top. Line i7 is connected to an ethylene storage tank (not shown) through a chiller (not shown) adapted to reduce the temperature of the ethylene to the desired low level, eg. 15 F.

A salt water reservoir, illustrated schematically at A2l, contains an aqueous solution of a salt that reduces the freezing point of the solution to below the temperature to which the cavern fl@ is to be maintained. For example, the salt Water can be a 25 weight percent solution of calcium chloride in water. Reservoir 2l is connected by line 22 having valve 23 and line 24 to a chiller v25 capable of reducing the temperature of the salt solution to the desired low level, c g. 15 F. Chiller 25 is connected through line 26 to the salt water inlet line 111.

in establishing the desired temperature conditions for cavern lli, salt water is rst pumped from the reservoir 2l through chiller 25 into line ll and the cavern is vented through valve 2.5i in line i9. This permits the cavern to be filled with cold salt water. When the cavern is full, valve 29 is closed and pump `lo in line 14 is star-ted at a rate to withdraw salt water at the same rate at which it is being introduced to the cavern. The withdrawn salt water circulates through line 27, valve 28, line 24 and chiller Z5, whence it returns to the inlet line Il. Circulation of the cold saltwater in this manner is continued until the temperature of the cavern walls has approached the salt water temperature. The cavern then is ready for introduction ofthe liqueed ethylene. This is done by pumping the ethylene, chilled to a temperature of say 15 F., through pump 18 and line 17. At the same time pump 13 is stopped and the salt water which is displaced by the ethylene is passed through valve 29 and line 3? into the reservoir. Introduction of the liquid ethylene is stopped before all of the salt water is displaced from the cavern so that a body of salt water, as indicated at 3l, is maintained in the cavern. Thereafter, to maintain the desired low temperature in the cavern, -cold salt Water is fed 'through line 1l into the salt water layer and an equivalent amount of salt water is removed through line 14. By heat transfer through the liquid ethylene to the salt water layer, the desiredy low temperature of the cavern walls Y is continuously maintained. Any water in fissures in the cavern Wall will become frozen so that seepage of water intothe cavern will be prevented.

Whenever it is desired to remove ethylene from the cavern, pump 16 is stopped and valve 15 is clos-ed while introduction Vof cold salt water through line lli is continued. In ithis manner the liquid ethylene can be displaced from the cavern through line 17, valve 2@ and line 19.

I claim:

'1.V In the storage of `a normally gaseous water-insoluble material in an underground cavern the Walls of which normally have a temperature above the critical temperature of said material, the method of storing said material in liquid phase which comprises lling the cavern with a cold aqueous salt solution having a freezing point substantially below said critical temperature, circulating such salt solution at a temperature between its freezing point and said critical temperature through the cavern until the temperature or the cavern walls has approached the temperature of the salt solution, displacing salt solution from the cavern by pumping in said normally gaseous material as a liquid chilled to a temperature below its critical temperature until lthe level of the salt solution has dropped substantially, stopping such displacement before the'level of the salt solution has reached the bottom of the cavern thereby leaving a body of salt solution in the cavern, and circulating cold salt solution to and withdrawing salt solution from said body of salt solution to maintain the cavern at a temperature below said critical temperature.

2. Method according to claim l wherein the temperature of the salt solution in the cavern at all times is below the freezing point of water, whereby any water in ssures adjacent the cavern wall is maintained in frozen condition.

3. Method according toV claim :l wherein said normally gaseous material is ethylene.

4. Method according to claim l wherein the solute of said aqueous salt solution is calcium chloride.

References Cited in the tile of this patent UNITED STATES PATENTS 2,932,170 Patterson Apr. ll2, 1960 2,961,840 Goldtrap Nov. 29, i960 

1. IN THE STORAGE OF A NORMALLY GASEOUS WATER-INSOLUBLE MATERIAL IN AN UNDERGROUND CAVERN THE WALLS OF WHICH NORMALLY HAVE A TEMPERATURE ABOVE THE CRITICAL TEMPERATURE OF SAID MATERIAL, THE METHOD OF STORING SAID MATERIAL IN LIQUID PHASE WHICH COMPRISES FILLING THE CAVERN WITH A COLD AQUEOUS SALT SOLUTION HAVING A FREEZING POINT SUBSTANTIALLY BELOW SAID CRITICAL TEMPERATURE, CIRCULATING SUCH SALT SOLUTION AT A TEMPERATURE BETWEEN ITS FREEZING POINT AND SAID CRITICAL TEMPERATURE THROUGH THE CAVERN UNTIL THE TEMPERATURE OF THE CAVERN WALLS HAS APPROACHED THE TEMPERATURE OF THE SALT SOLUTION, DISPLACING SALT SOLUTION FROM 